There is needed in many contexts to be able, in a simple and rapid manner, to disinfect sanitary facilities and washing areas and the equipment located there. Such needs exist, for example, at hospitals, nursing facilities, public baths, sports centers etc. It is highly desirable in such contexts that the disinfectant can be supplied to the sanitary facility by a simple operation, for example employing a permanently installed shower apparatus already available in the sanitary facility. In such applications, use is often made of permanent installations which are connected to the water mains. In these installations, the disinfectant is added and mixed with water which is taken from the water mains. In such instance, there is a certain risk that, in the event of partial vacuum in the water mains, the disinfectant is sucked back into the mains. Since the water mains also distribute drinking water, even a minor risk of such resuction is, naturally, unacceptable. In order to prevent resuction of, for example, hot water, through a faucet or from a water heater, use is normally made of one or more non-return valves. Since a resuction of disinfectant may have very serious consequences, the safety afforded by one or more non-return valves is considered insufficient.